97 silverado having misfire while pulling trailer

[Rickyrick]

This only happens when i pull my work trailer, no trailer truck runs fine. When i have the trailer i get to about 45-50 mph and then the motor acts like a spark plug wire is off. Let off the gas it'll smooth back out but try to pass 50 mph it does it again. Parts I've changed...distributor cap, dist is only a year old. Plugs, wires, air filter, fuel filter, o2 sensor. Has good fuel pressure. Only code that comes up is a misfire for cylinder 3, or cylinder 5 depending on the day.

[Rickyrick]

Forgot to put the engine. 5.7 vortec

[bcrooms]

Mine is having similar issues, but has actually given code P0300

[silveradosid]

check for intake gasket leaks